When an electrophysiological signal is sensed, the sensed signal can be contaminated with interfering noise from various sources. For the example of electrocardiogram (ECG) signals, sources of the noise can be a combination one or more of the following: electromagnetic radiation (e.g., about 50 Hz or 60 Hz depending on the country and harmonics) which include radio transmission, electrical wires, fluorescent lights; motion artifacts and baseline drift (e.g., ranging from about 0 Hz to about 20 Hz); electromyographic noise (e.g., ranging from about 20 Hz to about 200 Hz); and other electrophysiological equipment (e.g., generally ranging from about 0 Hz to about 2000 Hz). Given that the diagnostic frequency spectrum of ECGs can range from about 0.5 Hz to about 100 Hz, the presence of the noise can make a physician's analysis and diagnostic interpretation of the data difficult, especially when the sensed signals are low voltage signals (e.g., less than about 50 μV).
A common and simple post-analysis method to remove contamination noise is to use a digital filter. For example, a band pass digital filter can eliminate both high and low frequency noise and only allow the desired frequencies to pass. However, the frequency response of a narrow frequency band filter may also remove physiologically relevant frequencies of the ECG signal. Another approach is to use a signal averaging technique which is commonly used in signal-averaged electrocardiography. The method may reduce small amounts of interfering noise, but cannot process large amount of noise or variations within the ECG signal.